USB Drives Not Working In VirtualBox on Windows 7 Host
USB drives plugged into Windows 7 host would be sensed by VirtualBox (they show up under the Devices menu but would never actually get captured). In my case this applies to an Ubuntu 12 LTS guest, but others appear to have the same issue with other guest distros. Attempts to capture the USB device in the guest would result in a VirtualBox error about the device being busy. Furthermore, if the device in unplugged once the error occurs and then re-plugged, a second identical device shows up under the VirtualBox device menu (this is just a symptom of the problem and doesn't avoid the issue in anyway).
Note that this problem assumes the USB filters in the VirtualBox settings are set up correctly to allows the device to be captured. If you haven't done this, or don't know what this means, then this problem probably doesn't apply to you cause you haven't set up USB filters (...and probably need to).
In case this post disappears, here it is: